The nerdfighters have made real-life versions of many things in it, like go-fast bars and the shopping list, as partially documented in thisVlogBrothers episode.
Also, Omnictionary was once made into a real website; it was a wiki for John Green and the activities and history of Nerdfighteria, although it's now disbanded.
What Could Have Been: In the first draft, Q and the crew finally catches up to Margo in Kashmir (yes, that war-torn region in Asia), whereupon Margo is involved a freak earthquake. The end of the book features Q searching for her in the rubble.
There was a brief idea to have Ansel Elgort play Gus, Margo's security guard friend, as a nod to his role in The Fault in Our Stars, but it was thought it would be too confusing. Instead he plays Mason, the gas station clerk.